1. Field of the invention
The present invention relates to a solid-state imaging system, and, more particularly, to a technique which can be effectively used for, for example, a device for taking out picture element signals formed by a photoelectric transferring element via a MOSFET and to which a sensitivity varying function is added.
2. Description of the Related Art
There is a known MOS solid-state imaging system to which a sensitivity varying function is added. This known solid-state imaging system includes a vertical scanning circuit for varying the sensitivity for the purpose of controlling a time difference from the reading vertical scanning circuit by making this vertical scanning circuit perform the vertical scanning operation. As a result, the storage time for the photodiode is controlled. Such a MOS solid-state imaging system having a sensitivity varying function of the type described above can be exemplified by, for example, a MOS solid-state imaging system disclosed in Japanese Patent Application No. 61-179902, corresponding to U.S. Pat. No. 4,870,493.
The imaging system using the solid-state imaging device performs the sensitivity control by making 1 H (horizontal scanning time) a minimum unit. The sensitivity control circuit in the solid imaging device controls its sensitivity at the most preferable level from the minimum sensitivity immediately after the power source is turned on. As a result of the thus-performed sensitivity control, the image is changed as image fade-in phenomenon occurs, causing the image to become observed easily.
However, when imaging starts under a relatively poor illuminance, a response problem arises in that a relatively longer time, for example 8 to 9 seconds, needs to be taken since the change from the minimum sensitivity to the maximum sensitivity is caused to be changed by 1 H unit.
It is an object of the present invention is to provide a solid-state imaging system in which the response can be improved.
The above and other objects and novel features of the invention will more fully appear from the following description and the accompanying drawings.